The present invention relates to a control device for a pump apparatus for a vehicle, the pump apparatus being driven by a d.C. motor.
FIG. 1 shows an example of conventional control devices of this kind. In FIG. 1 the control device includes a vacuum pump 1 for supplying a negative pressure to a vacuum booster or a reservoir (not shown) of a brake system of a vehicle (not shown), a d.c. motor 2 for driving the vacuum pump 1, and a control circuit 3 for the d.c. motor 2. The control circuit 3 includes a power transistor 31 connected in series with the d.c. motor 2, a driver transistor 32 for controlling the power transistor 31, bias resistors 33 and 34 for the respective transistors 31 and 32, a diode 35 for absorbing surges, and a base-emitter resistor 36. The control device further includes a normally closed pressure switch 4 which is provided inside of the vacuum booster or reservoir for detecting the negative pressure generated by the vacuum pump 1. The switch 4 is opened when the negative pressure reaches a predetermined value. The control device further includes a battery 5 for supplying current to the control circuit 3 when a keyswitch 6 is closed.
With the conventional control device described above, prior to the time the engine (not shown) is started, the negative pressure in the vacuum booster or reservoir may be as low as atmospheric pressure, and the contacts of the pressure switch 4 will be in the closed state. When the keyswitch 6 is closed under this condition, current flows from the battery 5 through the bias resistor 33 to the base of the power transistor 31 since the driver transistor 32 is then in a nonconducting state. As a result, the power transistor 31 turns on, actuating the d.c. motor 2 to thereby drive the vacuum pump 1. Thereupon, the pressure in the vacuum booster or reservoir is decreased gradually. That is, a negative pressure is created.
Then, when the negative pressure reaches the predetermined value, the pressure switch 4 is opened, and hence current flows from the battery 5 through the keyswitch 6 and the bias resistor 34 of the drive transistor 32 to turn the latter on. Therefore, the power transistor 31 becomes nonconductive, causing the d.c. motor 2 and hence the vacuum pump 1 to be de-energized. Consequently, as the negative pressure in the vacuum booster or reservoir decreases, the contact of the pressure switch 4 is closed, again driving the d.c. motor 2. This operation is repeated to maintain the negative pressure in the vacuum booster or reservoir at the predetermined value.
In the above-described conventional control device, however, the on-off operation of the pressure switch contact is often so frequent that the so-called hunting phenomenon may be caused, which reduces the lifetime of the device.